The present invention relates to a heat reflecting film, more particularly, to a heat reflecting film which is applied to a window glass of a vehicle, a building or the like for reflecting near infrared heat rays in sunlight.
Conventionally, in order to reduce the cooling load of an air conditioning system in the vehicle or the building, attempts have been made to apply such a film to intercept the heat rays in sunlight on the surface of the window glass. The above described films are formed of metal such as gold or aluminum. However, the metallic film has problems in that the visible rays do not sufficiently penetrate therethrough and that the heat rays are absorbed by the metallic film so that the absorbed heat is partially transmitted to the interior of the room.
In order to solve the above problems of the metallic film, an interference filtering film composed of a plurality of dielectric layers has been proposed and practically employed.
This interference filtering film can penetrate the visible rays and reflect only the infrared rays by selecting the optical thickness of each layers thereof.
The conventional interference filtering film is composed of two kinds of dielectric layers having a different refractive index, which are alternately piled up. And the optical thickness (refractive index X thickness) of each layer is precisely adjusted into .lambda./4 wherein .lambda. is wavelength of the heat rays to be reflected by the interference filtering film.
The layer having a high refractive index is formed of zirconium oxide (ZrO.sub.2), titanium oxide (TiO.sub.2) cerium oxide (CeO.sub.2) or the like. And the layer having a low refractive index is formed of magnesium fluoride (MgF.sub.2), silicon oxide (SiO.sub.2), cryolite (Na.sub.3 AlF.sub.6), cerium fluoride (CeF.sub.2) or the like. The above described material is adhered to the window glass by sputtering, vacuum evaporating, spraying or the like so that the formed film has a predetermined optical thickness.
In order to improve the reflecting effect of the heat reflecting film, it is required to increase the number of the layers and to precisely adjust the optical thickness of each layer.
However, it is difficult for the present level of application technique to form a film composed of a large number of layers of which the optical thickness is not scattered.
When the thickness of each layer is widely scattered, the transmittance of the visible rays is largely affected causing uneveness in color.
When the number of the layers is decreased, the scattering of the optical thickness of each layer can be decreased but high heat rays reflecting efficiency cannot be obtained.
The conventional heat reflecting film has another problem as described below.
Generally, the thickness of each layer of the heat reflecting film is expressed by the formula: optical thickness/refractive index. The refractive index (n.lambda.) for the heat rays having wavelength of .lambda. has been conventionally used in the above formula.
The refractive index for the incident rays changes in accordance with the wavelength thereof and particularly, greatly changes as the wavelength is decreased. And the refractive index also changes in accordance with the material of each layer of the film.
Therefore, when the thickness of each layer of the heat reflecting film is determined by using the refractive index for the heat rays as described above, the obtained thickness greatly differs from such a thickness as to transmit the visible rays.
As a result, the peak reflectance of the film for the visible rays is large.
This phenomenon is particularly observed when the incident angle of the incident rays to the normal line of the surface of the heat reflecting film is large.
Accordingly, one object of the present invention is to provide a heat reflecting film (hereinafter referred to as "heat rays reflecting film") which is suitable for applying to the window glass of the vehicle or the building in order to decrease cooling load of an air conditioning system therewithin.
Another object of the present invention is to provide a heat rays reflecting film having heat rays reflectance substantially equal to that of the conventional interference filtering film and having improved transmittance of the visible rays.
Still another object of the invention is to provide a heat rays reflecting film of which the peak reflectance for visible rays is reduced in order that the objects outside of the window can be seen without any change to their color.